GROUND WATER IN NORTH-CENTRAL TENNESSEE

16 GROUND WATER IN NORTH-CENTRAL TENNESSEE
Thus, the Unaka Mountains, being composed largely of dense crystal­
line rocks, have persisted through relatively long periods with high
relief and rugged contour. On the other hand, the alternation of hard
and soft and of soluble and insoluble beds in the faulted and closely
folded rocks of the Valley of East Tennessee has facilitated erosion.
Consequently this district has a low average altitude, although it is
characterized by alternating valleys and ridges of moderate relief and
mature contour, which reflect faithfully the geologic structure of the
underlying rocks. Furthermore, the massive and very slightly de­
formed Pennsylvanian sandstones on which the Cumberland or
"Cretaceous" peneplain was cut, although raised to relatively high
altitude by subsequent uplift, have resisted erosion so that extensive
remnants of the peneplain have been preserved virtually undissected.
These remnants constitute the Cumberland Plateau. The underlying
Mississippian limestones, however, offered much less resistance to
mechanical and chemical denudation and, wherever the protective
cover of Pennsylvanian rocks was breached, were reduced quickly to
the profile of erosive equilibrium. The surface thus formed, an
erosion terrace of very slight relief from 700 to 850 feet below the
Cumberland peneplain, is the present Highland Kim plateau. After
renewed upwarping, mechanical and chemical denudation trenched
the Highland Rim plateau and cut another terrace the Nashville
Basin on the Ordovician limestones in the central part of the State.
This erosion terrace is between 200 and 600 feet below the Highland
Rim plateau. These featureless terraces cut on the flat-lying lime­
stones of the Interior Low Plateaus province contrast sharply with the
diverse land forms on the poorly consolidated sand, clay, and silt that
compose the Gulf Coastal Plain. Although the Coastal Plain is in
general a westward-sloping plain, it ranges from hilly or rolling to
gently undulating and reflects the differences in resistance of the rocks
that immediately underlie its surface. As a result of their character­
istic land forms, rock formations, and geologic structure, these physio­
graphic districts have more or less distinct modes of ground-water
occurrence. The modes that prevail in the Nashville Basin and the
northern part of the Highland Rim plateau are brought out in the body
of this report.
The Mississippi Alluvial Plain, unlike the other major physiographic
divisions of the State, is a constructional stream plain that has not
suffered general attack by destructive agencies.
HIGHLAND RIM PLATEAU
The interstream tracts of the Highland Rim plateau define a very
slightly undulating plain (see pi. 2, A), which entirely surrounds the
Nashville Basin and constitutes about 65 per cent of the region covered
by this survey. (See pi. 1.) Formerly, however, it extended over